PT Unknown
AU Krug Dr., I
TI Magnetic Proximity Effects in Highly-ordered Transition Metal Oxide Heterosystems studied by Soft X-Ray Photoemission Electron Microscopy
PD 04
PY 2008
LA en
AB Abstract:

In this thesis, the magnetic proximity effect (MPE) in
highly-ordered transition metal oxide (TMO) heterosystems
composed of single crystals of ferrimagnetic
(FIM) Fe3O4 and thin antiferromagnetic (AF) NiO layers has been
investigated by Photoelectron Emission Microscopy using polarized
soft x-rays (XPEEM).  The systems have been prepared in-situ by
Molecular Beam Epitaxy on single crystalline Fe3O4 substrates
polished to various crystallographic surface orientations and
conditioned by Ar sputtering and annealing in O2 background.  The
magnetic order was determined by vectorial magnetometry
exploiting XMCD and anisotropic XMLD for single crystalline
systems of cubic symmetry.  Two major contributions to the MPE
were identified: First, short-ranged interfacial exchange
interactions create an ultrathin zone of altered magnetic
structure near the interface. Second, long-ranged magnetoelastic
interactions lead to a change of the magnetic structure on a
larger scale, affecting the whole NiO adlayer.  The influence of
directional lattice strain on the magnetic order via
magnetoelastic coupling was studied by means of samples with
different crystallographic interface orientations. The strain
appears to affect the AF stacking-directions in NiO as well as
the coupling behaviour at the NiO/Fe3O4 interface. Additionally,
the in-plane bonding anisotropy of the films leads to variations
of the uncompensated magnetization induced in the NiO AF layer
via exchange coupling. It was found, that the uncompensated
magnetization resides directly at the interface, and the bulk of
the NiO layers is compensated. XMCD sum-rule analysis of a NiO
wedge on Fe3O4 (110) revealed extremal values for the Fe and Ni
orbital moments for 1ML, possibly related to the reconstruction
of the interface layer to NiFe2O4. Temperature-dependent
measurements of the XMD contrast reveal lowered critical
temperatures for both NiO and Fe3O4 due to finite size effects
and interfacial coupling.  Fits of the theoretically expected
XMLD contrast to profiles of the exchangeinduced AF domain walls
yielded a wall structure consistent with a simple coherent
in-plane rotation model of the NiO spin-axis. In
magnetically-annealed samples, the anisotropy of the
Fe3O4 (110)/NiO interface was found to be altered, leading to
non-crystallographic easy-axes. In a simple picture, the effect
may be explained as a superposition of bulk and interfacial
magnetocrystalline and magnetoelastic anisotropies. A
highly-ordered Fe3O4 (110)/NiO[51Å]/Co[15Å] trilayer-system was
found to exhibit the same composite anisotropy as mentioned
before, and in addition a possibly roughness-driven perpendicular
interlayer coupling between Co and Fe3O4.
ER